Multi-spot illumination for wafer inspection

Abstract

Illumination subsystems for multi-spot wafer inspection are provided.

What is claimed is: 
     
       1. An illumination on subsystem configured to provide illumination for wafer inspection, comprising:
 a diffractive optical element configured to separate an illumination light beam into multiple light beams; 
 a compensating diffractive optical element positioned in the path of the multiple light beams, wherein the compensating diffractive optical element has the same diffraction angle as the diffractive optical element but reverse in diffraction order; and 
 one or more refractive optical elements positioned in the path of the multiple light beams exiting the compensating diffractive optical element and configured to separately and simultaneously focus each of the multiple light beams to a wafer for inspection. 
 
     
     
       2. The illumination subsystem of  claim 1 , wherein the illumination light beam is provided by a pulsed laser and has a central wavelength of 266 nm. 
     
     
       3. The illumination subsystem of  claim 1 , wherein the bandwidth of the illumination light beam is equal to or greater than 10 pm. 
     
     
       4. The illumination subsystem of  claim 1 , wherein the compensating diffractive optical element diffracts only one diffraction order of each of the multiple light beams. 
     
     
       5. The illumination subsystem of  claim 1 , wherein the compensating diffractive optical element is a transmission grating that satisfies the Bragg condition. 
     
     
       6. The illumination subsystem of  claim 1 , wherein the compensating diffractive optical element is a blazed transmission grating. 
     
     
       7. The illumination subsystem of  claim 1 , wherein the compensating diffractive optical element includes multiple gratings having different pitches. 
     
     
       8. The illumination subsystem of  claim 1 , wherein the one or more refractive optical elements comprise a lens array configured to separately and simultaneously focus each of the multiple light beams to the wafer. 
     
     
       9. The illumination subsystem of  claim 1 , wherein the multiple light beams are spatially separated from each other at the compensating diffractive optical element and at the one or more refractive optical elements. 
     
     
       10. The illumination subsystem of  claim 1 , wherein the one or more refractive optical elements comprise a lens array configured to separately and simultaneously focus each of the multiple light beams and relay optics configured to separately and simultaneously relay the focused multiple light beams from the lens array to the wafer. 
     
     
       11. The illumination subsystem of  claim 1 , wherein the compensating diffractive optical element is a one-dimensional chirped grating, and wherein the one or more refractive optical elements comprise a cylindrical lens.